The prior art includes alkali metal oxide inorganic binders, some of which are derived from silicon dioxide in the form of a sol or silicates of potassium oxide, sodium oxide or lithium oxide in the form of silicate solutions. With respect to the basic SiO.sub.2 -K.sub.2 O binders, a 5.3:1 silicon dioxide to potassium oxide mol ratio was the highest achievable without resulting in a soft coating. Sometimes even a 4.8:1 mol ratio was difficult to achieve when silicone was added to the binder to promote adhesion and to provide for easy mixing with zinc and aluminum particles, which were added as sacrifical elements for the protection of ferrous metals or aluminum alloys. Occasionally, however, sludging would sometimes result from the addition of the silicone.
Other prior art paint compositions utilized such ingredients as lithium hydroxide (LiOH) as the fluxing agent in the synthesis of inorganic alkali metal silicates in conjunction with silicon dioxide in the form of a hydrogel to attain the desired silicon dioxide to alkali metal oxide mol ratio. The hydrogel solution is fundamentally unstable with, for example, sodium oxide, without the addition of the lithium hydroxide which, if added, becomes a mixture of Si-O.sup.- -Li.sup.+ and SiO.sup.- and Li.sup.+ as separated entities in solution. In the context of this invention, instability refers to the characteristic of the hydrated silicon-oxygen tetrahedral network or the tendency of some crystalline form of the silicon dioxidealkali metal to spinoidally separate out. The lithium has the benefit of providing a more water insoluble paint after drying for a period of one day. However, use of lithium in the manufacture of silicates and sols provides binders, which when made into paints, dry to form softer coatings with often tenuous adhesive characteristics. Potassium silicate binders which proceed more slowly to insolubility offer harder coatings and improved adhesion compared with lithium silicates.
The prior art has also demonstrated how siliconsilicate binders containing potassium hydroxide as the fluxing agent can be manufactured reliably into a hydrogel sol to attain a mol ratio of 5.3 without sludging by the silicone or spinoidal decomposition during the shelf life of the binder. See, for example, my prior U.S. Pat. No. 4,162,169. This process also yielded hydrogel sol binders with a mol ratio of about 6, but with limited shelf life prior to undergoing spinoidal decomposition.
Since silica hydrogels contain up to 65% water, the manufacture of alkali metal hydrogel solutions and sols with solids contents greater than about 23% is impractical. To achieve greater solids contents, silica gel must be used. Of all of the alkali metal ions (Li, Na, K, Rb, Cs), only lithium and potassium possess the capability of promoting the manufacture of silicate binders with mol ratios greater than 5.0 which are stable. Lithium has the capability of suspending fully hydrated silica to a mol ratio of 6.4, whereas potassium can only promote the formation of such binders in the presence of aliphatic silanols. Lithium accomplishes this function by virtue of its electric field strength; potassium accomplishes the same function by virtue of its intermediate field strength, which is intermediate between (Li, Na) and (Rb, Cs), and its compatibility with aliphatic trisilanols. Thus, lithium forms bonds of the form ##STR1## where (H.sub.2 O) represents a water sheath and .fwdarw. a polarized electric field, while the potassium bond is of the form ##STR2## with negligible polarization. Without the silanol, lithium therefore provides more stable high mol ratio silicate binders than potassium by virtue of its closer association with the ##STR3## moiety and a more tightly bound water sheath, which serves to prevent condensation of the siloxy groups. Condensation can only occur after ##STR4## or in a pseudo fashion whereby the water sheath is dispelled viz: ##STR5## This grouping once formed is not readily solvated because of the high field strength of the lithium ion. Since potassium is already ionized from the siloxane group, reactions (2) and (3) can occur more readily. Potassium silicate binders can be produced to imitate those of lithium through the use of aliphatic silanols, particularly methyltrimethoxysilane (CH.sub.3 Si(OCH.sub.3).sub.3, provided the silanol is the first ingredient added to the silicate starter solution. By so doing, the dominate reaction is EQU CH.sub.3 Si(OCH.sub.3).sub.3 .fwdarw.CH.sub.3 Si(O.sup.31 K.sup.30).sub.3.
Because the bond between the methyl group and the silicon atom is polarized toward the silicone atom, this inductive effect introduces the possibility of the following reactions occurring: EQU CH.sub.3 Si(OH).sub.3 --CH.sub.3 Si(OH).sub.2 O.sup.- +H.sup.+
and EQU CH.sub.3 Si(OH).sub.3 --CH.sub.3 Si.sup.+ (OH).sub.2 +-OH
The latter possibility provides a cationic moiety which behaves more like lithium than potassium. This behavior provides a basis from which mol ratios to about 6.4 for potassium silicate-silicone binders can be formed to imitate those at the limit of lithium silicate which is also about 6.4.
The present invention provides an inorganic binder which exhibits superior corrosion protection capabilities over organic binders and which is especially valuable with respect to protection of ferrous metals and aluminum alloys in a salt environment.